With recent and rapid developments of information communication industries, electronic devices have become smaller, lighter, slimmer and more portable, which increases a demand for batteries having higher energy density as a driving power source of an electronic device. Among such batteries, lithium ion secondary batteries satisfy this demand, and numerous studies towards improvements are now in progress actively. A lithium ion secondary battery includes a cathode, an anode, an electrolyte and a separator that provides a passage for lithium ions moving between the cathode and the anode. When lithium ions are intercalated into or disintercalated from the cathode or the anode, the lithium ion secondary batteries generate electric energy by means of a redox reaction.
The initial design of such lithium ion secondary batteries uses a lithium metal having a high energy density as an anode and also uses a liquid solvent as an electrolyte. However, lithium ion secondary batteries of this type have a short life cycle due to dendrite formation. In order to solve this problem, there have been developed lithium ion secondary batteries using a carbon material capable of absorbing a large amount of lithium ions as an anode instead of lithium metal and also using an organic liquid or solid polymer as an electrolyte.
The lithium ion secondary batteries using a carbon material as an anode active material employ a mixture of a cyclic carbonate, such as ethylene carbonate and propylene carbonate, and a linear carbonate, such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate, as an electrolyte solution. However, such a non-aqueous electrolyte solution disadvantageously exhibits a low lithium ionic conductivity at low temperature. For this reason, the lithium ion secondary batteries has low charging/discharging cycle efficiencies, which has a negative influence on low temperature charging/discharging properties.
Thus, there have been continuous studies attempting to improve high rate charging/discharging characteristics of a secondary battery by using a non-aqueous electrolyte solution. For example, Japanese Patent No. 3,032,338 and No. 3,032,339 disclose a non-aqueous electrolyte solution using a ternary system of a cyclic carbonate, linear carbonate and linear ester compound in order to improve high rate charging/discharging characteristics and low temperature charging/discharging cycle efficiencies.
However, such a ternary-system non-aqueous electrolyte solution exhibits the problem of increased swelling at a high temperature, though the low temperature charging/discharging characteristics are partially improved. Thus, there is a need for improving charging/discharging efficiency in lithium ion secondary batteries and solving swelling problem at high temperatures.